Method of thawing and cooking food



July 23, 1968 METHOD OF THAWING AND COOKING FOOD Original Filed June 4 1962 R. L. CAMPBELL POWER SUPPLY LEvEL SETTING 4| ,42 58 56 RF! POWER ID FILTER SUPPLY SERVO D I I; 43 1 {5? s2 49 0 k /45 oouPLlNg NET OSCILLATOR TRIPLER QB JEF FIG. 4

3 Sheets-Sheet 1 I .6 |2 |4 I R F COUPLING OSCILLATOR N ET RICHARD L. CAMPBELL INVENTOR ATTORNEY July 23, 1968 R. L. CAMPBELL 3,394,007

METHOD OF THAWING AND COOKING FOOD Original Filed June 4 1962 5 Sheets-Sheet 2 FIG. 2

FIG. 3

RICHARD L. CAMPBELL INVENTOR ATTORNEY.

July 23, 1968 R. L. CAMPBELL METHOD OF THAWING AND COOKING FOOD 3 SheetsSheet 5 Original Filed June 4, 1962 m OE RICHARD L. CAMPBELL INVENTOR.

1 "o ATTORNEY United States Patent 3,394,007 METHOD OF THAWING AND COOKING FOOD Richard Lincoln Campbell, Watertown, Mass. (R0. Box 482, Lompoc, Calif. 93436) Continuation of application er. No. 277,398, May 1, 1963, which is a division of application Ser. No. 199,824, June 4, 1962. This application May 19, 1966, Ser. No. 551,466

1 Claim..(Cl. 99-4) ABSTRACT OF THE DISCLOSURE A method for the defrosting and warming to serving temperatures of food such as cheese sandwiches and TV dinners is disclosed wherein defrosting is effected by a high voltage predominantly electrostatic phase shifting to an induction mode whereby the remaining heating to serving temperature is largely through the mechanism of induction heating. The relatively high voltages required for electrostatic heating are generated by a resonant wrapper applied to the food by the supplier and proportioned to match the particular food item.

This application is a continuation of copending patent a plication Ser. No. 277,398 filed May 1, 1963, now abandoned, which was a division of parent application, Ser. No. 199,824 filed June 4, 1962, now abandoned. This invention relates to the electronic defrosting and cooking of food. More particularly it relates to a method for speeding the thawing and subsequent heating of food whereby uniformity of heating, or planned nonuniformity of heating, may be accomplished by cooks us ing the invention.

Electronic cooking equipment is well known in the art. Examples of this kind of equipment are disclosed in M. M. DeBell, Jr., Patent No. 2,937,259 and in D. B. Haagensen Patent No. 2,920,174. Induction heating of tissue by radio frequency current commonly called diathermy has long been recognized in the medical arts. Dielectric heating of lossy insulating materials is also well known in the artas represented by C. E. Ellsworth Patent No. 2,923,801.

Electronic heating of food which ordinarily is an inhomogeneous assembly' of materials having various dielectric and conductive properties, has been accomplished by use of microwave energy whereby both dielectric and induction heating are combined. The microwave systems, however, have suffered from difficulties relating to the distribution of energy within the oven or working cavity. The above mentioned DeBell and Haagensen patents represent prior art attempts to solve these dlfilCllltiCS.

In microwave ovens of early design, ultra-high frequency electromagnetic energy coupled into a microwave cavity establishes a resonant multimode field intensity pattern in the cavity, similar to standing waves exhibiting regions of high and low intensities. The energy is generally inserted directly from the open end of a wave guide or by a coaxial transmission line and short exciting probe. Lossy dielectric material such as food or certain plastics placed in the oven are heated according to their position in relation to the resonance pattern. This results in uneven application of heat. One method for providing for Patented July 23, 1968 more even distribution has been to utilize a fan with rotating blades within the enclosure, the fans acting as reflectors which stir the resonant pattern thereby shifting the standing wave minima, and this way promoting uniformity of heating. In another method, the item to be heated is rotated on a turntable to achieve the same result. Frequency variations may also be utilized to spread the energy; or multiple feed probes and other complex transmission and radiation structure may be used to control these variations in heating intensity. Infrared heating may be superimposed on the electronic heating to brown the food, in a way which modifies the heating pattern. In spite of these various techniques, the cooking of frozen foods or the defrosting and warming of pre-cooked meals by the microwave ovens has proved to be unsatisfactory when high-speed operation is desired. The source of this difliculty lies in the marked difference in the electrical properties of ice and water. When both contain a substantial ionic content, as is usually the case with the constituent water in food, conductivity increases markedly upon thawing. When a deep frozen food package is heated in a microwave oven, one portion of the package is bound to thaw first due either to inhomogeneity of the food or of non-uniformity of the microwave field. Thereafter electromagnetic energy is much more strongly absorbed by the fluid portion and the conducting portions tend to shield the unmelted parts from the electrostatic field. The penetration of the induced current is limited by the skin effect. Skin depth is inversely proportional to the square root of the conductivity of the medium and also to the square root of the frequency. (See Terman Radio Engineers Handbook, First Ed., McGraw-Hill, p. 34.) The temperature of the melted part is rapidly raised to the boiling point. There may be dehydration and even burning in spots on the surface while unmelted blocks of ice remain close-by in the package, electronically shielded by the conducting fluid. If the heat is supplied SLOWLY enough, the ice is melted by conduction and the hot spots may be kept below the boiling point until all of the ice is melted. Thereafter, the package is more homogeneous electrically and is more uniformly heated to the desired serving temperature. Depth of penetration remains a problem but to a lesser degree.

It is a feature of the present invention to employ relatively lower frequency radiowaves for the defrosting and cooking of foods thereby substantially increasing the penetration of the radio energy. The wave length of the energy used is considerably greater than any dimension of the oven chamber in which the food is defrosted and heated.

A particular feature of the invention by which the lower frequency of the invention may be utilized efficiently is a resonant wrapper comprising a pattern of conductors cut and proportioned to distribute energy within the food in a predetermined manner. It is a further feature of this invention that this resonant Wrapper may be embodied in the disposable package in which the food is packed by the processor. In another form of the invention, the wrapper is of a more permanent nature, such as a glass or temperature resistant plastic housing which serves as a form for conductor pattern attached (by printed circuit or other means) to the wrapper. Other features and advantages of this invention will be apprehended from the following specifications and annexed drawings of which:

FIGURE 1 is a block diagram of the basic system,

FIGURE 2 shows an output tank circuit of a typical system embodying the invention,

FIGURE 3 is a prospective drawing of a typical coupling wrapper in accordance with the invention,

FIGURE 4 is a block diagram of the system of the invention with automatic loading control, and

FIGURE 5 illustrates a wrapper tray for a TV dinner.

The design of electronic system for cookery is quite sensitive to radiation requirements promulgated by the Federal Communications Commission. Unnecessary radiation in any case is unlawful; but the Commission has provided a number of frequencies (13.56 mc., 27.12 mc., and 40.68 me.) which are set aside for industrial electronics, diathermy, industrial induction heating, dielectric heating, etc. Alternatively, other unassigned frequencies may be used if sophisticated mechanisms are employed to hold radiation levels to the required limits of microvolts per meter at a distance of 1000 feet. A range of frequencies between 2 megacycles per second and 100 megacycles per second are potentially useful for the present invention. In this frequency range, the most practical form of radio frequency generator is the high vacuum high-voltage electron tube oscillator and power amplifier. Sometimes the oscillator is used without power amplifier. This is illustrated in FIGURE 1 wherein the block 11 represents an interference filter through which house-current is supplied, the block 12 represents the oscillator, the block 13 represents a power supply which converts house current to the required high direct voltages needed for the power tubes of the oscillator. The output of the oscillator 12 is transmitted by the coupling network 14 to the output solenoid 15. Solenoid 15 is proportioned to permit the insertion within its turns of the food to be warmed in a wrapper 33.

The solenoid 15 is made to resonate with a capacitor. Its coil comprises an equivalent one or two turns of inductance distributed to provide a substantially uniform strong radio frequency magnetic field in the working chamber of the oven. The solenoid is contained in a compartment which combines the shield 10, the intermediate shield 16, and suitable doors to provide access to the working compartment while minimizing spurious radiation. The design of metal parts is such as to minimize the loss of energy to the shielding structure.

As a particular example of the invention and as the preferred embodiment thereof I offer the example of apparatus for the defrosting and warming of sandwiches. Such apparatus is suitable for home use or for snack-bar use. As a particular example of a food portion that may be defrosted and warmed is an American cheese sandwich on white bread. The constituents of such a sandwich are as follows: I

Commercial white flour bread (Tip-Top bread) two slices 41.5

Oleomargarine 2.5 Salad dressing 2. American cheese, 2 slices 52.5

Total 98.5

Using a generator at 40.68 megacycles with an input power availability to the power stages of 500 watts, I prefer an output solenoid on a form with rectangular insertion 5 inches long by 4 and /2 inches wide by 1 and inches high as shown in FIGURE 2. The coil and capacitor assembly consists of two coil sections 21 and 22, each of two turns, connected in parallel by straps 24 and 25 capacitors 27 and 28, totalling approximately .00005 microfarad are connected between the straps 24 and 25 to complete the resonant circuit. Tuning to 40.68 megacycles is accomplished by adjusting the position of the turns, after which they are doped in place. Radio frequency power is'supplied from a generator coupling unit through appropriate connections 30 and 31.

Use of flat strips isappropriate for the operating frequency to reduce skin effect losses see Te'rman, opus cited at p. 33 ff. FIGURE 3 illustrates the coupling network wrapper 33 which I prefer for this cheese sandwich. It is 5 inches long by 4 inches wide by 1% inches high. It comprises a single layer solenoid 34 of adhesive aluminum foil tape A inch wide wound with a pitch of /2 inch per turn on a paper form as above dimensioned and into which I place the sandwich in the deep frozen state at approximately zero degrees Fahrenheit temperature.

In operation, a few hundred volts is impressed across the output coil 15. The resonant step-up. of .voltage and the turns ratio results in some thousands of volts across wrapper coil 34 which applies an intense electrostatic field to the frozen sandwich.

With this arrangement a time of 60 seconds'is -typically required to defrost the sandwich and additional 20 seconds required to bring it up to serving temperature.

By comparison a deep-frozen cheese sandwich placed in the output tank circuit without a coupling wrapper after 5 minutes is still unthawed, measuring 20 degrees F. An important feature of the interrelationship between the output solenoid 15 and the coupling wrapper solenoid 34 is that the coupling is close, that is, well above the so called critical coupling level as understood in the art, see Terman, (op. cit., Radio Engineers, Handbook, First Edition, page 154 if, McGraw-Hill Book Co., N.Y.). The coupling is also well below unit. Accordingly when the sandwich is in its frozen state, and the loading of the wrapper is light, its Q factor is high and there is a great resonance step-up of voltage. The wrapper is, in effect, a Tesla coil and many thousands of volts per inch of field strength at radio frequency is applied to the frozen food. This high-intensity electric field is effective to thaw the frozen article. When thawing is effected the conductivity is usually great enough that. the remaining heating to serving temperature is largely through the mechanism of induction heating.

The operation of the invention as above described tends to maintain a flow of power into the load by shifting from an electrostatic to an induction mode of heating as the conductivity of the food changes. This effect alone is not sufficient to insure efiicient heat transfer for various load conditions, particularly when there is a change of state of constituent water.

For increased efliciency and shorter cooking time means for adjusting the coupling network during the defrosting-cooking cycle are required as illustrated in FIG- URE 4. These means can be made automatic by well known control system principles.

In the apparatus of FIGURE 4, as in FIGURE 1, utility power is tapped through an interference filter 41, and rectified in a power supply 42 from which direct current is supplied through a shunt 4.3 to the radio frequency power amplifier 45. The amplifier 45 is excited by an oscillator 49 and a triplingstage 50. The output power is transmitted by a coupling network 52 to the load 53 similar in general design and function to the output tank circuit of FIGURE 2 with wrapper and food.

Ideal matching of the load 53 to the power amplifier 45 requires setting of bothimpedance level and power factor. But in practice while the power transfer depends markedly on tuning (power factor) a small mismatch in impedance levels has a relatively unimportant influence. Accordingly a satisfactory degree of match between amplifier/l5 and load 53 can sometimes be obtained by adjusting a single parameter. Tests made with many dif ferent foods have demonstrated that a satisfactory degree of control can be maintained by adjusting a variable capacitor-55in the coupling network 5 2 to'tmaintainamplifier wplate currentat the desired level. It is convenient to operatethe system detuned on the high "frequency'side of the resonance curve so that an increase'in plate current is related to an increase in capacitance. Capacitance,'in

this range, is a single valued function of plate current. At the start of a defrosting cycle the capacitor 55 is set to minimum value. Thereafter the servomechanism 56 adjuststhe capacitance by a mechanical linkage 57 to maintain plate current measured at the shunt '43 at the value determined by the level setting input 58. A unique advantage of the invention is the control thereby made possible over the distribution of "heating-power in the oven. The invention provides for a greatly increased absorption of energy by food within the coupling Wrapper, while the output coil provides a field much less effective by itself. In the example cited above, a cheese sandwich was warmed in about one and one-half minutes, while the output coil alone would raise the temperature only about 20 degrees the sandwich remaining frozen. Thus, food properly coupled to the energy may be brought out piping hot in a period of time where without coupling ice cream would not even be melted.

FIGURE represents a four compartment TV dinner tray 151 and cover 152. The tray has four compartments 153A, 153B, 153C, and 153D. The cover 152 has corresponding compartments 154A, 154B, 154C, and 154D. The tray is closed by folding the two halves together along the line 155. Both tray 151 and cover 152 have patterns of conductors applied to them by which energy is coupled to the food.

The pattern of conductors in each of the four compartments is different. By these differences the amount and character of the heating energy applied to each portion is individually determined. The four compartments formed by closing the cover are designated compartment A, compartment B, compartment C, and compartment D. Compartment A encloses its food portion between electrostatic shields 161 and 162. Heating in this compartment is very inefficient, making it suitable, for instance for bringing ice cream up from freeze temperature to serving condition. Compartment B is wrapped with a coil that incorporates metal strips 163, 164, 165, 166, and 167 which are joined by the closure of the tray to form a coil. Compartment C features two electrodes 170 and 171 in tray and cover respectively. These electrodes are supplied with a high voltage field by interconnection with the metal strips 172 and 173 of compartment D, with which they form a coil. The resulting heating in compartment C is therefore primarily electrostatic, useful for brown and serve rolls. The electrode 171 is made somewhat transparent so that infrared browning may be employed.

The arrangement of compartment D provides for deep heat primarily by induced conduction. This is done primarily by reducing the ratio of inductance to capacitance L/C in the coupling wrapper and maintaining high electrical quality in a portion, at least, of the capacitive element.

Foil wafers 176 and 177 insulated from the strips 170, 171, 172, and 173, by thin insulating sheets, 178 and 179 introduce a substantial capacitative element in the circuit. This increases the circulating current, and reduces current dependence upon dielectric losses in the load.

Any of several good insulating materials may be used for the sheets 178 and 179. A transparent material such as the polyester resin known as Mylar is shown. This is shown for convenience in illustration. A plastic coated paper is a possibility, as is combination of foils and insulation into a suitable laminate.

Attached to the tray is a card extension 180 on which a pattern of conduction strips 181, 182, 183, 184, and 185 is shown. The pattern of these strips uniquely identify this package and in code contain any necessary instructions to the oven. This card is marked in the same way that the conductors are applied to the tray, and this step preferably is carried on with the deposit of the conducting strips in the tray as a single operation. The card provides for automated control of various packaging processes. It helps to eliminate possible inconsistency between label and package or between food contents and either of them.

The oven may be arranged with a receptacle for the card, so that it activates the oven controls automatically when the package is inserted. It is preferred to detach the card and insert it into a separate slot in the oven. The primary reason for this preference is to provide utmost simplicity and Washability for the oven compartment itself.

From the above it will be apparent that there is a very great number of possible package variations by which foods may be defrosted, warmed, and cooked.

For example, the conductors making up the resonant circuits may be applied to the outside of the container. Application to the inside is shown in FIGURE 5 largely for convenience in illustration. Application of the foils 176 and 177 have the effect of increasing the capacitance and the circulating currents in the central portion of compartment D. In some cases it is desired to reduce the intensity of heating in the central portion. To accomplish this, the capacitance may be reduced between the coil and the food. One expedient for accomplishing this is to interpose between coils and food a foamed plastic portion of the container. Containers comprising an inner sheet, and an outer sheet of paper or plastic separated by a layer of foamed plastic are desirable in that control of designed electrical properties is facilitated.

This invention for the processing of food raises a number of novel problems in food processing. One immediate problem is the necessary concern for conductivity and dielectric properties of food, which have nothing to do with nutrition. New production controls may be necessary. The technique to be used for a given package and a given food within the package must be determined. The pattern of conductors that is optimum for a particular product will depend upon the power available from the generator, the operating frequency, as well as the electrical properties of the food and of other foods to be cooked with it.

These problems will be worked out, largely by a cut and try method by a central laboratory. Maximum economy can be achieved if the packaging, precooking, defroster design, holding temperature, and other variables are under central coordination.

At this time it is not known whether the great majority of food products can be eifi-ciently served using only one frequency of energy. The choice of a particular frequency or a pair of frequencies would be made on the basis of market research and then economic balancing of all factors.

Notwithstanding the changes in the equipment, or its power and its frequency, the design of packages may be effected using the teaching of this invention, whereby induction and electrostatic heating in proper proportions may be employed for the rapid and efiicient defrosting and heating, and cooking of food. Such modifications and expedients will be recognized as falling within the. scope of the invention as particularly pointed out by the following claim.

I claim:

1. The method of preparing a serving of food comprising the preliminary steps, in either order, of:

packaging said serving in a wrapper having a pattern of conductors in the form of metal strips proportioned and arranged to convert a portion of a strong electromagnetic induction field into a predominantly electrostatic field, and

deep-freezing said serving, followed by the condition.-

ing steps of:

applying to said packaged and frozen serving a predominantly electromagnetic radio frequency field at a wave length substantially greater than the dimensions of said wrapper, said field being converted by said strips into a predominantly electrostatic field and applied for suffiicent time and with sufficient intensity to thaw said serving, and

j applying thereafter a predominantly induction radio suflicient time to heat to serving temperature, said electrostatic and induction fields being controlled to maintain an efficient flow of power into said serving While shifting from an electrostatic to an induction mode of heating as the conductivity of the food changes. I

References Cited UNITED STATES PATENTS 

